1. Field of the Invention
The present invention relates to a positive photoresist composition which exhibits high sensitivity and definition, and improved focal depth range properties and underexposure margin. The term "underexposure margin" used herein means a ratio of Eop/Es, where the exposure to exactly reproduce a pattern in accordance with predetermined dimensions by a mask pattern at an illumination intensity at substrate of 500 mW/cm.sup.2 is defined as the exposure Eop (ms) and the minimum exposure to give an isolated pattern at an illumination intensity at substrate of 500 mW/cm.sup.2 is defined as the exposure Es (ms).
2. Description of the Related Art
In the manufacture of ultralarge-scale integrated circuit (Ultra LSI) where a high definition of not more than half amicron, in particular, of not more than 0.35 .mu.m is required, there are demands on photoresist compositions which can form a resist pattern satisfactory in definition, underexposure margin, focal depth range properties and having a good shape.
By way of illustration, Japanese Patent Laid-Open No. 6-167805 discloses a positive photoresist composition containing a quinonediazide ester having a tetra- to hepta-nuclear linear polyphenol skeleton as a photosensitizer. This conventional composition has high definition but remarkably low underexposure margin of 1.0 to 1.2 due to its extremely high .gamma. value, and can hardly form a pattern particularly at positions with low exposures (underexposure side).
In the exposure step, therefore, the formation of a resist pattern is blocked when the exposure is shortened for only several milliseconds relative to the optimum exposure (exposure Eop) or when thickness of a resist film becomes only several hundred nanometers thicker at locations corresponding to steps of a substrate.
In addition, a material containing the aforementioned photosensitizer hardly forms an isolated pattern when the focus of a light source in the exposure step shifts to the plus side (i.e., the light focus is in the substrate side from the resist surface) in the formation of an ultrafine resist pattern of not more than 0.35 .mu.m. FIG. 5 demonstrates changes in the formation of resist patterns with shifts of the focus, where (a) indicates the case using the aforementioned photosensitizer, and (b) indicates the case using another photosensitizer having no tetra- to hepta-nuclear linear skeleton. FIG. 5 demonstrates that the pattern formations in both cases are similar when the focus shifts to the minus side from the zero point where the focus is not shifted, but the use of the aforementioned photosensitizer fails to form any pattern when the focus shifts to the plus side, which is inherent to this photosensitizer.
As a result, the photosensitizer is disadvantageous in that it is narrow in focal depth range and cannot form a resist pattern of the predetermined dimensions when the distance between a light source and a substrate shifts in the exposure step or when a resist film is formed on a stepped substrate and hence the surface of resist film is uneven due to steps of the substrate.
Further, Japanese Patent Laid-Open No. 8-245461 discloses a high-definition positive photoresist composition containing a quinonediazide ester of the specific compound which is disclosed in the present invention as a compound of the formula (Ia) as a photosensitizer, and its Example 1 describes an embodiment using a photosensitizer containing the above quinonediazide ester and a quinonediazide-tetraester of 4-(4-hydroxy-2,5-dimethylbenzyl)pyrogallol.
This composition exhibits high definition but extremely low underexposure margin of 1.0 to 1.2 due to its very high y value, and can hardly form a pattern particularly at positions with low exposures (underexposure side), as in the above-mentioned Japanese Patent Laid-Open No. 6-167805.
In addition, no isolation pattern is formed when the focus of a light source shifts to the plus side in an exposure step in the formation of an ultrafine resist pattern of not more than 0.35 .mu.m (FIG. 5), and hence the composition has narrow focal depth range and has disadvantageous as in Japanese Patent Laid-Open No. 6-167805 mentioned above.
Furthermore, Japanese Patent Laid-Open No. 63-115162 mentions a positive photoresist composition having improved film residual rate after development, which is obtained by using a quinonediazide ester of the specific compound which is disclosed in the present invention as a compound of the formula (IIIa). The composition is inferior in definition and cannot form a pattern having satisfactory underexposure margin and focal depth range properties in the formation of an ultra fine pattern of not more than 0.35 .mu.m.
Japanese Patent Laid-Open No. 60-121445 also discloses a positive photoresist composition using, as a photosensitizer, a quinonediazide ester of a compound, which is disclosed as the compound of the formula (IVa) in the present invention. The composition is inferior in definition and cannot form a pattern having satisfactory underexposure margin and focal depth range properties in the formation of an ultrafine pattern of not more than 0.35 .mu.m.